Micropitting is an unexpectedly high uniform rate of fatigue wear. It occurs in rolling sliding Elasto Hydrodynamic Lubrication (“EHL”) contact during the first million rotation cycles of machine life. The affected gears typically have a gray matte finish on the contact surfaces with microscopic examination revealing a network of cracks and micropits 10 to 20 micrometers in diameter. This type of failure has been a chronic problem with large gearboxes including the gearboxes used in the wind turbine industry. Micropits coalesce to produce a continuous fractured surface with a characteristic dull matte appearance variously called gray staining, frosting, or, in German, graufleckigkeit when applied to gears. The related term for the phenomenon in bearings is peeling or general superficial spalling. Micropitting is generally, but not necessarily exclusively, a problem associated with heavily loaded case carburized gearing.
The progression of micropitting may eventually result in (macro)pitting, or it may progress to a point and stop. Although it may appear innocuous, such loss of metal from the gear surface causes loss of gear accuracy, increased vibration and noise, and other related problems.
Methods for measuring micropitting of gears have been developed at the FZG Institute in Munich more than a decade ago. See “Influence of the Lubricant on Pitting and Micro Pitting. Resistance of Case Carburized Gears—Test Procedures” Winter, H; Oster, P. AGMA Technical Paper 87 FTM 9, October 1987. The FZG approach was subsequently developed into a procedure sponsored by the FVA association in Germany and formally published in 1993. See “FVA-Informationsblatt Nr. 54 I-IV:Testverfahren zur Untersuchung des Schmierstoffeinflusses auf die Entstehung von Grauflecken bei Zahnradern” FVA-Nr. 54/7 Stand Juli 1993.
The FVA 54/7 procedure has become the industry standard for assessing industrial gear lubricant micropitting-resistance performance. The method uses the FZG power-circulating equipment that has two separate stages. First, a progressive loading test or stage test in which the pinion or smaller of the two gears in a set must be dismounted and rated after each 16-hour load stage from load stage 5 through load stage 10. Then the second side of the gear set is run in a stage test involving load stages 5 through 10 each 16 hours long with fresh oil. This is followed by the endurance test in which the gear is run with the same oil charge as the second stage test for a total of six 80-hour periods starting at load stage 8 for the first 80 hours, and then finishing at load stage 10 for subsequent 80 hour periods. Inspections are performed between each period. The inspections assess micropitted area of the pinion tooth flanks, pinion weight loss and the deviation of profile form. Tooth profile measurement is carried out through use of a profilometer. The sensing tip is moved from tooth tip to root and the topography is fed into a computer program. The before and after test measurements are compared and the difference reported as “profile deviation”. The damage load stage is reached when the profile deviation exceeds 7.5 μm.
Mobilgear Synthetic HydroCarbon-Xtra Micro Protection or (“SHC XMP”) sold by ExxonMobil Corporation in Fairfax Va., was commercialized in 1998 as a micropitting resistant industrial gear oil. The primary market for this lube is the wind turbine industry. Mobilgear SHC XMP was very successful in use with one exception. That exception is the superior level of performance demanded by builders today in the, Graufleckigkeit Test “GFT” FLS greater than 10 Class High. GFT Class High is a rating requiring a FLS greater than 10. Mobilgear SHC XMP 320 provides a FLS equal to 10 high. Currently, only the BP Castrol Optimol Synthetic A 320 product claims this equivalent level of micropitting performance.
In the last several years, there has been a number of key equipment builders in this sector that are starting to require the highest level of performance in the FVA 54 Micropitting test of FLS greater than 10. A high FLS greater than 10 high rating require less than 7.5 microns of gear tooth profile deviation in the FVA 54 Micropitting test at the end of stage 10 loads. At the current time, there are no known hydrocarbon based lubes that consistently give this level of performance. Accordingly, there is a need for a lubricant that provides a consistent FVA 54 Micropitting test result of FLS greater than 10 high. The present invention satisfies this need by providing a novel combination of base stocks that give the desired performance.